Post-consumer aluminum scrap processing allows the aluminum metal to be recovered and returned to re-create the original consumer products. Recovery rates are extremely important as the losses must be replaced with primary metal which has a specific energy rate of ten times the specific energy rate for recycled metal. The original consumer products have a variety of specialized coatings (e.g., coatings on interior surfaces of cans to protect the aluminum from reacting with liquids borne by the cans, thermoplastic or thermoset coatings on exteriors of cans, etc.) which remain intact on the scrap and must be removed prior to melting of the aluminum metal. Failure to completely remove the coatings prior to melting the scrap results in metal loss (12-13%) from an uncontrolled thermite reaction and dross generation.
When aluminum scrap is returned to be recycled, it is usually contaminated with volatile organic coatings (VOCs). There are many different types of aluminum scrap, originating from manufacturing processes or consumers. Post-consumer waste comes in many different forms, but is generally grouped into three categories, which are defined by their VOCs as a percentage of weight: Low (<5% by weight), Medium (5%-25% by weight) and High (25%+ by weight). These coatings vary in composition, but can comprise, for example, one or more of oil, scrap, lacquer and/or plastic, amongst other materials. All of these coatings must be removed before sending the aluminum to the melting furnace(s) for further recycling processing. Conventionally, removal of the VOCs from used beverage containers (UBC) and other kind of scrap is primarily accomplished using rotary kilns or single-chamber batch (charge) furnaces.
Well-known methods for addressing VOCs in aluminum scrap are set forth in the following patents. U.S. Pat. No. 4,654,088, titled “Decoating of aluminum scrap,” sets forth a method of removing organic coatings from scrap aluminum by using a gas-permeable conveyor to pass scrap metal through a pyrolysis zone where an oxygen-containing hot gas is directed downwardly through the scrap metal, the hot gas being at a temperature to raise the upper surface of the scrap to a temperature in the range of 500°-600° C., with the temperature and rate of supply and oxygen content of such gas and the rate of travel of the scrap being adjusted to ensure that a reaction front at which the organic material is pyrolyzed and residual carbon is burned travels from top to bottom of the bed within the pyrolysis zone and to allow the scrap to be retained within the pyrolysis zone for up to 10 minutes after the reaction front reaches the bottom of the scrap.
U.S. Pat. No. 5,405,428, titled “Decontamination and/or surface treatment of metals,” relates to a process of decontaminating metal (e.g., aluminum or aluminum alloy) scrap contaminated with organic material, the process involving heating the scrap in a fluidized bed of solid particles fluidized by a fluidizing gas to a decontaminating temperature high enough to consume the organic material but below the melting point of the metal. The heating step is carried out in the presence of a protective material which protects the aluminum or aluminum alloy against substantial oxidation while the scrap is held in the bed at the decontaminating temperature. U.S. Pat. No. 6,227,847, titled “Apparatus and process for removing volatile coatings from scrap metal,” sets forth a system for removing volatile coatings from scrap aluminum including a kiln, a fan for generating an airstream, an afterburner for heating the airstream, and ducting for confining the airstream in a closed loop so that it circulates through the afterburner, the kiln and back to the fan in that order. The ducting includes a bypass duct into which a portion of the airstream is diverted at a diverter value, before being heated by the afterburner. This portion reenters the heated portion of the airstream downstream from the afterburner and serves to modulate the temperature of the airstream entering the kiln. The temperature where the airstream enters the kiln is maintained at a temperature hot enough to volatilize coatings on the aluminum, yet not hot enough to melt the aluminum. As it passes through the kiln, the airstream possesses a diminished oxygen content, so that the volatilized coating does not ignite. The fan responds to another temperature sensor located where the airstream leaves the kiln such as to vary the mass flow, so that the temperature of the airstream leaving the kiln likewise remains constant. A collector exists in the ducting, between the kiln and the fan, and should its surfaces become cool enough to condense the volatilized coatings on them, the system recirculates some of the heated airstream to the collector to maintain the airstream entering it above a prescribed temperature.
By way of example, one conventional process to remove organic materials (see, e.g., U.S. Pat. No. 6,227,847, supra) utilizes an IDEX® chip dryer, shown in FIG. 1, comprising a rotary kiln to heat the scrap material to remove the organic materials through volatilization, using high temperature, low O2 gas, and an afterburner chamber in which the liberated organics are subjected to controlled incineration.
Common problems exist with the above methods such as, but not limited to, high energy intensity for the treatment process, incomplete removal of coatings, oxidation of the scrap metal surfaces, and environmental impact of further treatment required to clean the metal.